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name: creating-adts description: This skill should be used when the user asks to "create an ADT", "scaffold a discriminated union", "add a tagged union", "create Effect Schema union", or needs to create algebraic data types following the ADT library pattern with proper member files and barrel exports.

Creating ADTs

Scaffold ADT (Algebraic Data Type) unions with discriminated member types.

CRITICAL

Read the Conventions First

Before creating any ADT, read the convention documents:

• ~/.claude/docs/conventions/namespace-module.md — Core _.ts/__.ts pattern
• ~/.claude/docs/conventions/library-adt.md — ADT-specific patterns

Operations

Create ADT Union

Structure:

Copysrc/lib/<adt-name>/
├── _.ts              # export * as AdtName from './__.js'
├── __.ts             # exports member namespaces + union
├── <adt-name>.ts     # Union definition
├── <member-a>.ts     # Member type
└── <member-b>.ts     # Member type

Steps:

1. Create directory at src/lib/<adt-name>/ (kebab-case matching ADT name)
2. Create member files with tagged structs
3. Create union file importing members directly (NOT from __.ts)
4. Create __.ts exporting members as namespaces + union exports
5. Create _.ts pointing to __.ts
6. Add package.json imports and tsconfig.json paths

Member File Pattern (Effect Schema)

Copy// <member>.ts
import * as S from 'effect/Schema'

export class MemberName extends S.TaggedClass<MemberName>()('AdtNameMemberName', {
  // fields...
}) {
  static is = S.is(MemberName)
}

Naming Rules:

• Class name = member name (short, local): Versioned
• Tag name = fully qualified: DocumentVersioned

Union File Pattern

Copy// <adt-name>.ts
import * as S from 'effect/Schema'
import { MemberA } from './member-a.js' // Direct import, NOT from __.ts
import { MemberB } from './member-b.js'

export const AdtName = S.Union(MemberA, MemberB)
export type AdtName = typeof AdtName.Type

Barrel File Pattern (__.ts)

Copy// __.ts
export * from './<adt-name>.js'
export * as MemberA from './member-a.js'
export * as MemberB from './member-b.js'

Examples

LifecycleEvent ADT

Copy// added.ts
export class Added extends S.TaggedClass<Added>()('LifecycleEventAdded', {
  schema: SchemaRef,
  revision: Revision,
}) {
  static is = S.is(Added)
}

// removed.ts
export class Removed extends S.TaggedClass<Removed>()('LifecycleEventRemoved', {
  schema: SchemaRef,
  revision: Revision,
}) {
  static is = S.is(Removed)
}

// lifecycle-event.ts
import { Added } from './added.js'
import { Removed } from './removed.js'
export const LifecycleEvent = S.Union(Added, Removed)
export type LifecycleEvent = typeof LifecycleEvent.Type

// __.ts
export * as Added from './added.js'
export * from './lifecycle-event.js'
export * as Removed from './removed.js'

// _.ts
export * as LifecycleEvent from './__.js'

Consumer Usage

Copy// Import ONLY from namespace (_.js)
import { LifecycleEvent } from '#lifecycle-event'

// Access members via namespace
const added = LifecycleEvent.Added.make({ schema, revision })
const removed = LifecycleEvent.Removed.make({ schema, revision })

// Type check
if (LifecycleEvent.Added.is(event)) {
  // event is Added
}

Notes

• Union file imports members directly to avoid circular dependencies
• Consumer code imports ONLY from _.js, never from __.js
• Use .make() constructor from tagged classes, never manual object construction
• Tag names should be fully qualified (AdtNameMemberName) for global uniqueness
• See convention doc for Simple ADT Pattern (alternative when full structure is overkill)